H1N1 Flu (Swine Flu)

A New Influenza Virus

Novel influenza A (H1N1) is a new flu virus of swine origin that was first detected in April 2009. The virus is infecting people and is spreading from person-to-person, sparking a growing outbreak of illness in the United States. An increasing number of cases are being reported internationally as well.

It’s thought that novel influenza A (H1N1) flu spreads in the same way that regular seasonal influenza viruses spread; mainly through the coughs and sneezes of people who are sick with the virus.

It’s uncertain at this time how severe this novel H1N1 outbreak will be in terms of illness and death compared with other influenza viruses. Because this is a new virus, most people will not have immunity to it, and illness may be more severe and widespread as a result. In addition, currently there is no vaccine to protect against this novel H1N1 virus. The Centers for Disease Control and Prevention (CDC) anticipates that there will be more cases, more hospitalizations and more deaths associated with this new virus in the coming days and weeks.

Novel influenza A (H1N1) activity is now being detected through CDC’s routine influenza surveillance systems and reported weekly in FluView. CDC tracks U.S. influenza activity through multiple systems across five categories. The fact that novel H1N1 activity is now detected through seasonal surveillance systems is an indication that there are higher levels of influenza-like illness in the United States than is normal for this time of year. Most of the influenza viruses being detected now are novel H1N1 viruses.

CDC NOTE

This is a rapidly evolving situation and current guidance and other web content may contain variations in how this new H1N1 virus of swine origin is referred to. Over the coming days and weeks, these inconsistencies will be addressed, but in the interests of meeting the agency's response goals, all guidance will remain posted and new guidance will continue to be issued.